Extracorporal shock wave therapy (ESWT) seems to be a promising new tool for the treatment of chronic pain due to tendinopathies such as tennis elbow or a painful heel. Mechanisms of ESWT-induced analgesia are still unknown. One major system for controlling pain is the endogenous opioid system that could be the biochemical basis of the ESWT-effects.

The aim of the study was to investigate the possible influence of low energy ESWT on the endogenous opioid-system in the lumbar spinal cord of the rat.

Immunohistochemical analysis of the expression of opioids Met-Enkephalin (MRGL), and dynorphin (Dyn) were performed in rats treated either once with 1000 impulses or three times with 1000 impulses with two different energy flux densities each (0.04 and 0.11 mJ/mm²) at 4 or 72 h after ESWT.

No different immunoreactivity of MRGL and Dyn was seen after single ESWT treatment in comparison with the sham group. This result was not influenced by different energy flux doses or repetitive ESWT treatment. Met-Enk and Dyn expression was similar on ipsi- and contralateral side and was unchanged at later time points after ESWT treatment.

Low energy ESWT had no influence on the opioid-systems and therefore does not trigger this endogenous anti-nociceptive system under basal conditions. Furthermore these results show that low energy ESWT had no side effects on rat spinal cord (e.g. neuronal destruction or enhanced permeability of the blood brain barrier for leukocytes) even after the application of 3 x 1000 impulses with the energy flux density as high as 0.11 mJ/mm². Although applications in orthopaedics have outnumbered those in urology, there is no firm evidence of efficacy of ESWT in orthopaedics from well-designed randomised clinical trials and the molecular mechanisms of the of the anti-nociceptive effect of ESWT are still unknown.